1. Field of the Invention
This invention relates to apparatus for automated ultrasonic inspection of thick metal sections using an ultrasonic transducer, and more particularly to a method and apparatus for automatic calibration of the sensitivity of the system so that a flaw of constant size produces the same return signal independent of its position, and more importantly independent of its depth.
2. Description of the Prior Art
It is an accepted practice to nondestructively examine metals and other structural materials for flaws using ultrasonic signals. This technique is applied to thick sections that are used in boilers and pressure vessels in accordance with the American Society of Mechanical Engineering (ASME) Boiler and Pressure Vessel Code. The code sets forth the requirements for such examinations.
One of these requirements describes the calibration methods to be used prior to each inspection. A block of sample material, usually taken from the vessel itself during its construction, such as at a location where a passage is cut out to install nozzles through the vessel wall, is provided with three calibration holes drilled into it parallel to the surface at depths of 1/4, 1/2 and 3/4 of the thickness (T) of the sample. The sensitivity of the ultrasonic inspection system is then adjusted as a function of depth so that, for all three of these calibration holes, the same flaw indication signal is produced by the ultrasonic inspection system operating on the sample, nominally 50% of screen height on the instrument. This adjustment is referred to as a Distance-Amplitude Correction (D-AC).
The D-AC capability has heretofore been implemented with a controllable gain amplifier. The amplifier must correct for attenuation due to absorption which is an exponential function, beam spread which is an inverse distance-squared function, near-field effects, and other lesser contributions. The calibration is performed manually by adjusting a number of independent controls, typically from three to six controls, so that the signal from the three calibration holes give return signals at 50% of screen. For example, the gain control terminal of the D-AC amplifier may be connected to an analog exponential function generator synchronized with the round-trip time of the ultrasonic pulses reflected by the calibration holes at 1/4T, 1/2T and 3/4T.
The output of the D-AC amplifier is displayed while the function generator is adjusted to produce a constant amplitude signal for all three calibration holes at 50% of display screen height. This requires at least two adjustments in the exponential function generator to set the starting point and the rate of change of the function, and since the correction function is not a true exponential function, it is necessary to use two exponential function generators, each with the two adjustments just mentioned, and a combining (summing) circuit. The latter requires a fifth adjustment to set the ratio at which the two exponential functions are combined.
In general, the signals from the three holes cannot be displayed on the screen simultaneously, so that the interaction of the multiple adjustments on the return signals from the different holes cannot be seen. As a result, the process requires iteration between the controls and the signals. That is very time consuming (about 25% of total time devoted to inspections) and dependent upon the skill of the operator. Consequently, the procedure is not ordinarily continued once a minimally acceptable result is obtained which satisfies the code requirement.